I was trying to get a little more bass response on this one, and succeeded somewhat. The air resonance from the soundhole is about 1/2 way between low E and F, as you can see on the attached frequency response graph. That gives it a nice bottom end on the low notes, but the peak is quite narrow and the low frequencies start to drop off quickly above low G. Sounds pretty good - I'm happy with it.

Superb! just a lovely guitar, and I like the validation that the upper bout sound hole expands the bass response of the instrument. I love the headstock too, I always struggle with that part and innovation...

Thank for the nice comments! I hope it holds up to the string tension over time. I carved the top thinner and braced it lighter than I have previously dared to go. I think that paid off in sound quality (despite some conventional wisdom to the contrary), but only time will tell if the structure is adequate.

How thin is 'thin'? Also, do know what the top tap tone was before and after you tightened the strings?

The down load of the strings on the top compresses it, and this lowers the pitch of the 'main top' resonant mode; the tap tone you hear with the hole blocked. It's sort of like raising the tension on a banjo head, but in reverse, and is related to column loading.

Some time back Joshua Gordis, a teacher at the Naval Post Graduate Research Center, got in touch with me about the effect of string tension on the resonant modes of a guitar top. They have been using a resonance check as a non-destructive test of aircraft truss structures. Imagine a column that is fixed at both ends. If you tap on the side it will vibrate with a characteristic fundamental frequency, depending on the stiffness, mass, and length. If you load the column at the top to put in under compression, the resonant pitch drops. When the load is high enough to cause the column to buckle, the pitch goes to zero. Interestingly enough, the effect is linear: at half the load that would cause the column to buckle, the pitch will be halfway down to zero from whatever it was with no load. Thus, if you know the resonant pitches of all the members of a truss you can find the ultimate load that it will bear by simply loading it to half the lad you want, and seeing which members drop too much in pitch.

I checked this out by looking at the resonant modes of a flat circle of expanded styrene bead board. Putting a rubber band around it to compress the material did indeed lower the 'ring' mode pitch, but cutting the rubber band, and sticking it on with double sided tape brought it back up nearly to what it had been 'bare'. The weight of the band drops the pitch a bit, of course.

As it turns out, flat top guitars don't show any appreciable pitch change when loaded: compression in front of the bridge seems to be more or less balanced by tension behind it. Arch tops are another story: the 'main top' pitch does indeed drop when you tighten up the strings.

This is not a simple column, of course, so the relationship between the pitch change and the load may not be linear. Still, one would expect it to drop to something near zero when the top collapses. If it doesn't drop too much I take that as a sign that the top is not over loaded, at least in the short term. It could still cave in through 'cold creep', but that's mostly a matter of how you do the recurve, IMO.

Wow, that is interesting stuff. Opens up a lot of possibilities. I took Somogyi's course where we carved away top braces to near the "edge of the cliff". Going any further suddenly makes the top too floppy and the tap tone goes from resonant to sounding like cardboard. But the process is a blind because you don't know when you are nearing the edge until you have gone over it. I can see how this might let you know where you are and stop at a more comfortable point.

The guitars that I made with his method sounded wonderful for a couple of years and then they started losing it.

Alan Carruth wrote:How thin is 'thin'? Also, do know what the top tap tone was before and after you tightened the strings?

Unfortunately, I get carried away and forget to measure and record things as carefully as I should. But I know I carved most of the top to a uniform thickness of just over 1/8", except for a slightly thicker area around the bridge, and then did a lot of sanding. I'm guessing the thinnest part of the recurve ended up closer to 1/16", but that's just a guess. I was going for a somewhat springy feel in the top - not flabby, but not completely stiff either.

Here's a tap test I did after the box was closed and before the binding was done. You can see the low air resonance in about the same place, but the main top and back peaks are higher than in the final instrument, consistent with your observations.

The mass of the bridge, and even the tailpiece, can also drop the top pitch. The easy thing is to just slack off the strings until the bridge is just barely being held down, and check the tap tone then.

I've made arch top Classicals with tops at a uniform 3mm, but I've never gone that thin on a steel string. 1/16" sure does seem thin for the recurve, but you might get away with it if there's enough slope up from the edge. It will also help if you can keep the break angle over the bridge down; I'm not sure if it needs to be any greater than six degrees or a bit more.

Chasing the ever lighter arch top carve is what lead me to using CF for my soundboards. Your specs are about where I began to have tops fail. But, of course, CF doesn't sound like wood. I personally like it, but it is not the same. I have never taken a FFT snapshot of the guitars and tops. Maybe I should start so I can quantify what my ear hears.

I know this has been asked a thousand times, but what is the software and recording process you used to get the graph of the tap tone? I am assuming you are using the Gore books as your guide, right?

Archtop people seem to fall into two camps - lighter tops, lower bridge angle, lower tension strings, lighter bridges, lighter bracing vs heavy tops, big bridges, medium or heavy gauge strings, big bridge angles, and "drive the top" philosophy. My guitars wake up with lighter strings and less everything, I have never found an improvement with the "heavy, hard, big" approach. Unless maybe you plan to try to take on a 15 piece big band unamplified, and beat the crap out of it.

My thickness estimates are not very reliable. I'm much more the intuitive rather than metrics-driven kind of builder. And as D'Aquisto pointed out, the measurements are going to be different for every piece of wood anyway, so you just have to develop a feel for it. I'm pleased that with this guitar, I at least moved in the direction I intended to go.

I just plug a vocal microphone into my audio interface, hold the guitar on my lap in playing position, mute the strings and tap on the bridge. SPAN has an option that freezes the maximum amplitude for each frequency in the tap tone, resulting in a graph like the ones I posted. Interestingly, each guitar has a "signature" response that really doesn't change much with variations in mike placement or tapping technique.

The information you get from this is of limited value. I don't think anyone claims they can tell what a guitar will sound like just by looking at an FFT chart. Much of the perceived timbre results from the higher harmonics, and FFT charts just don't tell you much about those. It's most useful for the low frequencies. I do have Gore's books, and they contain much useful information, but he doesn't deal with archtops at all, so I wouldn't call that my guide.

An interesting thing I discovered with this guitar (at least I think it's interesting) is that I can get a strong helmholtz resonance from the body by blowing over the sound hole, just like you would with a jug. Of course I immediately tried this with my round-hole flattop guitars, and it didn't work at all. I suppose that's due to the size of the hole, and maybe distance from the edge. If I had more air, it might work.